In most applications of radar, both noise and unwanted signals are present due to other radars or radio transmitters in the area. An important consideration is how to distinguish the wanted return signal from both noise and unwanted signals. In theory, the transmitter could radiate so much energy that the returned signals would be much larger than any noise or unwanted signals. However, that would only work for one radar in any given area, so it is not a practical option. Other means of recognizing the wanted return signal have been devised.
Traditionally, a radar signal consists of a burst of 100 to 1000 cycles of a fixed frequency sine wave. As an example, if the carrier frequency is 333 MHz, then one cycle is 3 nanoseconds long, and the duration of a 100 cycle burst is 300 nanoseconds. An electromagnetic wave would travel approximately 100 meters during such a 300 ns burst. If the largest linear dimension of a target is small compared to 100 meters, then the target acts like a point scatterer and the returned pulse will have essentially the same amplitude versus-time variation as the transmitted pulse.
But, with pulses of very short duration (e.g., 1 ns), few targets can be considered as point scatterers. Consequently, such short pulses become heavily distorted. Distortions of the returned pulse, due to the finite extension of the target, are called the radar signature. In principle, the radar signature can provide information about such features as the shape of the target and the material composition of its surface. So on the one hand, a heavily distorted signal is good, in that it means that more information is available about the shape and composition of the target. But, on the other hand, it makes the return signal hard to recognize and selectively receive.
When the pulses incorporate a carrier frequency, reception is facilitated because the carrier frequency (i.e., the fine structure marking) can always be recognized, regardless of the amount of distortion. This is due to the fact that, the sum of any number of sinusoidal waves at a given frequency will always be a sinusoidal function at that same frequency, regardless of the amplitude and phase differences of the various sine waves. Hence, a sequence of bursts with enough sinusoidal cycles per burst to allow detection can be recognized by their carrier frequency regardless of the distortions. But, it is generally not useful to consider using fine structure marking, as provided by a carrier, with pulses that are 1 ns or less. This is because a burst of 1 ns or less would require a carrier frequency of 94 GHz or more to obtain 100 cycles within the burst, and electromagnetic waves at such high frequencies are too strongly absorbed by rain and fog to permit all weather operation. Even in clear weather, the absorption in the atmosphere limits the useful range of a 100 GHz signal to about 20 km. So new schemes must be devised.
Recently, radars have been developed both theoretically and experimentally that do not rely on bursts of a sinusoidal carrier. These radars typically use digital pulses with a duration of 1 ns or less and require special antennas such as shown in my U.S. Pat. No. 4,506,267, titled FREQUENCY INDEPENDENT ANTENNA, and which is hereby incorporated by reference. The return signals from such short duration pulses yields enormous amounts of information about the target. The various mathematical methods for extracting the information are known under the generic term "inverse processes." These methods are not a topic of this disclosure and are, therefore, not discussed herein. However, to make use of this information the radar must first have means that permit the selective reception of the wanted, distorted signal in the presence of both unwanted signals and noise. But, such selective reception is difficult. For instance, if the longest dimensions L of the target (e.g., the length or the wingspan of an airplane) is greater than .DELTA.tc/2 (i.e., where c is the velocity of light, and .DELTA.T is the duration of the pulse), then the radar signature is large, and distortion of the transmitted signal is great. In such a case, selective reception of the backscattered signal is difficult because it is so different from the transmitted signal. Yet, by carefully choosing the coarse structure of the radiated pulses, certain characteristics will remain unchanged, while others will be strongly effected by the target. The unchanged features can be used to aid selective reception, the changed features constitute the information about the target.
Hence, it is evident that for very short pulses, fine structure marking is impractical. Coarse structure marking can be relied upon to assist in reception instead. The remainder of this disclosure will show how marking with a fine structure can be replaced by marking with a coarse structure to enable the reception of heavily distorted signals, and also to permit long range all weather operation. This disclosure is a further improvement of concepts presented in the patent application "Detection of Radar Signals with Large Radar Signature in the Presence of Noise", Ser. No. 07/647,788, filed Jan. 30, 1991 by Henning Harmuth, which is hereby incorporated by reference.